Petrocephalus christyi Boulenger, 1920

Petrocephalus christyi Boulenger (1920): 11.

[Odzala field identification: Petrocephalus sp.7, OTU 7]

Images. Fig. 9 A, photo of a live specimen from Odzala, Fig. 9 B, photo of a preserved specimen from Odzala and Fig. 14, drawing of a syntype from Boulenger (1920), p.11. Additional photo of a syntype in Harder (2000).

Type material. Syntypes, MRAC 7145–7151 [five of seven specimens examined]. Democratic Republic of the Congo, Congo basin, tributary Lindi at Bosabangui [in the vicinity of Kisangani, estimated 0.53N, 25.19E], C. Christy coll. [Note that no holotype was designated in the original description of P. christyi.]

Other specimens. We examined two specimens from Odzala National Park and 16 additional specimens from the Sangha River and the Lower Congo River in the vicinity of the Pool Malebo (see specimen list provided in the section "additional material examined").

Syntypes Specimens Specimens from

(n=5) from Odzala Lower Congo and

(n=2) the Sangha River

(n=16)

Diagnosis. The following diagnosis is based on all examined specimens of P. christyi, regardless their geographic origins. Petrocephalus christyi is distinguished from all other Petrocephalus species in Central Africa by the following combination of characteristics. Dorsal fin with 22–24 branched rays. Anal fin with 25–29 branched rays. Eye large (HL/ED ≤ 4.2, range = 3.1–4.2). Mouth small (HL/MW ≥ 2.9, range = 2.9– 5.5, but see remarks). Nine to 13 teeth in the upper jaw, 17–22 teeth in the lower jaw. Melanin patterning consisting of two characteristic black patches: (1) a distinct round mark below the anterior base (first to the fifth rays) of the dorsal fin; and (2) a somewhat diffuse crescent-like mark, centered at the base of the caudal fin and extending onto the fleshy dorsal and ventral lobes of this fin. In the small number of individuals recorded so far, the EOD is of normal polarity, with four phases and a relatively slow initial rise.

Description. Morphometric and meristic data for the syntypes, non-type specimens from Odzala and specimens from Lower Congo are separately presented in Table 7. However, the following description corresponds only to the new material collected from Odzala National Park, except where separate reference is made to the Lindi River syntypes. Petrocephalus christyi is a medium-sized species within the genus (maximum SL observed in Odzala = 84.7 mm; maximum SL observed in the syntypes = 100.0 mm). Body ovoid, longer than high (2.7 ≤ SL/H ≤ 2.9, syntypes = 3.0–3.2) and laterally compressed. Head length between 3.2 and 3.5 times in standard length (syntypes = 3.5–3.7). Snout of intermediate size (5.3 ≤ HL/SNL ≤ 6.3, syntypes = 5.3–5.7) and round. Eye large (3.2 ≤ HL/ED ≤ 3.4, syntypes = 3.1–3.8). Mouth small (4.1 ≤ HL/ MW ≤ 4.3, syntypes = 4.7–5.5), sub-terminal, opening just under the anterior half of the eye. Teeth small and bicuspid, 9 or 10 in a single row in the upper jaw, 19–21 in the lower jaw. Dorsal and anal fins originate in the posterior half of the body, with pre-dorsal distance equal to pre-anal distance (SL/PDD = 1.6 and SL/PAD = 1.6). Dorsal fin with 22 or 23 branched rays (syntypes = 23 or 24). Anal fin with 25 or 26 branched rays (syntypes = 26 or 27). Scales cover the body, except for the head. Lateral line visible and complete with 35–37 (syntypes = 35–39) pored scales along its length. Fourteen to 15 scales (syntypes = 13 or 14) between the anterior base of the anal fin and the lateral line. Caudal peduncle relatively thin (2.5 ≤ CPL/CPD ≤ 2.6, syntypes = 2.5–3.0). Twelve scales around the caudal peduncle. Skin on head thick, turning opaque with formalin fixation, with numerous electroreceptors organized into three distinct rosettes. However, the Augenrosette (above the eye) is not as developed as it is in the other species of Petrocephalus exhibiting this rosette.

Live coloration (Fig. 9 A). Body uniformly white-silver, with the presence of two characteristic melanin marks: (1) a distinct round mark below the anterior base (first to fifth rays) of the dorsal fin; (2) a rather diffuse crescent-like mark, centered at the base of the caudal fin and extending onto the upper and lower fleshy lobes of this fin. The fins themselves are translucent.

Distribution (Fig. 1). Endemic to the Congo River basin. The type specimens were collected at the locality Bosabangui along the Lindi River, which is close to the city of Kisangani. Petrocephalus christyi seems to be rare in Odzala, where we only collected it in the main channel of the Lékoli River at night. In contrast, Petrocephalus christyi seems to be abundant in the vicinity of the Pool Malebo (Lower Congo).

Electric organ discharge (Fig. 9 C). Bearing in mind that recordings are only available for two specimens collected within Odzala, the EOD of this species does appear to be distinctive. The EOD exhibits normal polarity and is four-phasic with a large, negative-going P2 and smaller peaks, P1, P3 and P4 [see Lavoué et al. (2008) for definitions of these waveform landmarks]. Similar to the EODs of the Odzala population of P. microphthalmus, a relatively long, slow rise occurs during the initial part of the first head-positive phase in the EODs of both individuals of P. christyi. This rise toward P1 has a shoulder-like inflection point, which at magnified gain is seen to possess a small, negative-going local peak. Both the slow rise and inflection point are subtle features of the EOD; they are only visible in amplified traces. Unlike the EODs of P. microphthalmus, however, those of the two P. christyi individuals possess a prominent head-negative fourth peak (P4), the amplitude of which is 4.5 – 8.6 % of the waveform’s total peak-to-peak swing. A prominent P4 is also present in the EOD emitted by the only individual of Petrocephalus mbossou n. sp. ever recorded (see below), but otherwise a P4 seems to be rare in Petrocephalus. Based on 1.5% voltage deviations from baseline relative to peak-peak amplitude and at ambient recording temperatures, duration of the EOD is between 0.284 and 0.390 msec in P. christyi, falling in the range of many other Petrocephalus species. Statistics for waveform landmarks and other EOD measurements are provided by Lavoué et al. (2008). Electrocyte anatomy is presumed to be of type NPp based on the EOD waveform, but this has not yet been confirmed histologically.

Remarks. Specimens of P. christyi from Odzala appear to differ notably from the syntypes of P. christyi (from the Lindi River, Fig. 1) in only a single aspect of morphological variation: mouth size. The syntypes have smaller mouths than the Odzala specimens (SL/MW = 17.7–20.1 in the syntypes versus 13.6–14.3 in the Odzala specimens; see also the HL/MW ratios in Table 7). Interestingly, specimens from the Lower Congo in the vicinity of the Pool Malebo (Table 7) all have mouths similar in relative size to those of the Odzala specimens. The magnitude of divergence in mouth size relative to the syntypes, and the fact that divergence in this character is consistent for both Odzala and Lower Congo specimens, suggests that populations from these sites may represent a distinct species from the P. christyi type material. A sound evaluation of this possible difference requires additional material from other Congo River sites. Until such material becomes available, we conservatively assign the Odzala and Lower Congo specimens described herein to P. christyi.